The present invention relates to light-emitting devices, and more particularly to III-Phosphide and III-Arsenide based semiconductor light-emitting devices having improved light generating capability.
III-Phosphide and III-Arsenide material systems are suitable for the fabrication of light-emitting devices that generate light having photon energies which range, respectively, from the green to the red spectral wavelength regimes and from the red to the infrared wavelength regimes. III-Phosphide material systems include any combination of group III and group V elements with phosphorous. Example III-Phosphide materials include, but are not limited to, AlP, GaP, InP, AlGaP, GaInP, AlGaInP, GaInPN, and GaInAsP. III-Arsenide material systems include any combination of group III and group V elements with arsenic. Example III-Arsenide materials include, but are not limited to, AlAs, GaAs, InAs, AlGaAs, GaInAs, AlGaInAs, GaInAsN, GaAsSb, and GaInAsP.
III-Phosphide and III-Arsenide based light-emitting devices such as light-emitting diodes and laser diodes may be employed in a variety of applications such as street lighting, traffic signals, and liquid crystal display back-lighting. In such applications, it is advantageous to increase the flux (optical energy/unit time) provided by an individual light-emitting device. Unfortunately, the flux provided by conventional III-Phosphide and III-Arsenide based light-emitting devices can be limited by their conventional vertical geometry.
Referring to FIG. 1, for example, a typical conventional III-Phosphide or III-Arsenide light-emitting device 10 includes a III-Phosphide or III-Arsenide active region 12 disposed between an n-type conductive substrate 14 and p-type layer 16. P-contact 18 and n-contacts 20 are disposed on opposite sides of device 10. A suitable forward voltage applied across contact 18 and contacts 20 causes current to flow vertically through p-type layer 16, active region 12, and conductive substrate 14, and thereby causes active region 12 to emit light.
Typically, the flux provided by conventional light-emitting device 10 is reduced because a portion of the light generated in active region 12 is absorbed by conductive substrate 14. In some prior art devices light generated in active region 12 and incident on substrate 14 is absorbed because the band gap energy of substrate 14 is less than the photon energy of the generated light. In other prior art devices, in which the band gap of substrate 14 is greater than the photon energy of the generated light, substrate 14 still absorbs a portion of the generated light incident on it due to absorption by free-carriers in the substrate. These free carriers, typically generated by dopants, are necessary to support electrical conduction through substrate 14 between contact 18 and contacts 20.
Conductive substrate 14 is sometimes wafer bonded to the rest of conventional light-emitting device 10. The resulting wafer bonded interface lies somewhere between contact 18 and contact 20, and hence must be highly electrically conductive if the device is to operate efficiently. This conductivity requirement limits the material choices for the substrate. Also, the relative crystallographic orientations of the substrate and the device layer to which it is wafer bonded may be critically important to achieving low forward bias voltages (as explained in U.S. Pat. Nos. 5,66,316 and 5,783,477, both of which are incorporated herein by reference in their entirety). This complicates the manufacturing process for these devices. In addition, a conventional light-emitting device 10 having a wafer bonded substrate may also include additional layers adjacent to the wafer bonded interface in order to improve the interface""s electrical properties. Unfortunately, these additional layers can absorb light emitted by active region 12.
Some conventional light-emitting devices include layers which form a distributed Bragg reflector (DBR) located between active region 12 and absorbing substrate 14. In these devices, some of the light emitted by active region 12 is redirected away from substrate 14 by the DBR. Thus, loss due to absorption in substrate 14 is reduced. The reflectivity of the DBR, which is angle dependent, typically decreases for angles away from normal incidence. Consequently, the DBR typically does not reduce absorption in substrate 14 as much as desired.
The placement of contact 18 on top of conventional light-emitting device 10, opposite from contacts 20, also limits the flux provided by device 10. In particular, contact 18 typically either absorbs light generated in active region 12, or reflects it toward absorbing substrate 14. Moreover, contact 18 is typically electrically connected to a package or a submount with a wire bond. Such wire bonds, which can be mechanically fragile and may not handle large electrical currents, also limit the maximum flux that a conventional device can provide.
In addition, active region 12 is typically separated by substrate 14 from any heat sink on which conventional device 10 is mounted. Consequently, heat generated in or near active region 12 may not be effectively dissipated and the performance of conventional device 10 is degraded.
What is needed are III-Phosphide and III-Arsenide based light-emitting devices that do not suffer from the drawbacks of prior art devices.
A light-emitting semiconductor device includes a stack of layers including an active region. The active region includes a semiconductor selected from the group consisting of III-Phosphides, III-Arsenides, and alloys thereof. A superstrate substantially transparent to light emitted by the active region is disposed on a first side of the stack. First and second electrical contacts electrically coupled to apply a voltage across the active region are disposed on a second side of the stack opposite to the first side. In some embodiments, a larger fraction of light emitted by the active region exits the stack through the first side than through the second side. Consequently, the light-emitting semiconductor device may be advantageously mounted as a flip chip to a submount, for example.
A method of forming a light-emitting semiconductor device in one embodiment includes forming a structure including a stack of semiconductor layers overlying a host substrate, attaching a superstrate to a first side of the structure, removing at least a portion of the host substrate, and forming a first and a second electrical contact on a second side of the structure opposite to the first side. The stack of semiconductor layers includes an active region comprising a semiconductor selected from the group consisting of III-Phosphides, III-Arsenides, and alloys thereof. The superstrate may be attached to structure, for example, by bonding it to the stack or by growing it on the stack using conventional growth techniques. Consequently, the light-emitting semiconductor device may include a bonded interface and may include one or more bonding layers. The superstrate may be attached to the structure either before or after the host substrate is at least partially removed. The superstrate may be attached to the side of the structure from which the host substrate was at least partially removed, or to the side of the structure opposite to that of the host substrate.
The superstrate may be shaped to enhance the efficiency with which light is extracted from the device. A lens may be attached to the superstrate or the superstrate may be formed into a lens to further enhance light extraction efficiency.
Both the light extraction efficiency and the operating power level of light-emitting semiconductor devices disclosed herein may exceed those of conventional III-Phosphide based and III-Arsenide based light-emitting semiconductor devices. Hence, the disclosed light-emitting semiconductor devices may provide higher flux than conventional devices.
Also disclosed is an array of light-emitting semiconductor devices. Each of the light-emitting semiconductor devices comprises a stack of semiconductor layers including an active region, a superstrate disposed on a first side of the stack and substantially transparent to light emitted by the active region, and first and second electrical contacts disposed on a second side of the stack opposite to the first side. The light-emitting devices may be mounted as flip chips to a common substrate (such as a printed circuit board, for example) either directly or via submounts. The active regions of at least a subset of the light-emitting semiconductor devices include a semiconductor selected from the group consisting of III-Phosphides, III-Arsenides, and alloys thereof.